台西盆地烃源岩特征及天然气成藏条件

SOURCE ROCK CHARACTERISTICS AND GAS RESERVOIRING CONDITIONS IN WEST TAIWAN BASIN

台西盆地是中国东南海域新生代陆缘扩张形成的一系列北东向裂陷—坳陷盆地之一。该盆地主要发育3套烃源岩:①渐新统—中新统海相泥岩和海陆过渡相含煤岩系;②古新统—始新统浅海—半深海相碎屑岩;③白垩系海相泥岩和海陆交互相含煤层系。其烃源岩指标分析显示为好的生气源岩,并达到生气门限。这些烃源岩与上覆储、盖层形成有利生储盖组合,为天然气成藏提供了良好条件。在整体分析基础上,重点对该盆地西部海域的南日坳陷和澎湖坳陷进行了研究,认为白垩系和古新统—始新统烃源岩为其主要烃源岩,埋深接近2800m时进入大量生气阶段。通过对烃源岩成烃作用和含气系统的分析认为,以上2个坳陷具备天然气成藏的良好时空配置,是台西盆地天然气勘探的重要前景区。进一步推测分析认为,南日坳陷构造深部位可能具备根缘气成藏的有利条件。

The western Taiwan Basin is one of the NE-trending faulted/downwarped basins caused by the Cenozoic continental margin extension in offshore southeastern China. Three source rock sequences have been recognized from shallow to deep in this basin, including the Oligocene-Miocene marine mudstone and coal-bearing formations of transitional facies, the Paleocene-Eocene sand shale of shallow marine-bathyal facies, and the Cretaceous marine mudstone and coal-bearing formations of transitional facies. Source rock analysis shows that they are gas source rocks of high quality and have entered into gas window. These source rock intervals together with the overlying reservoirs and seals form favorable source-reservoir-cap rock combinations, providing good conditions for gas reservoiring. The Nanri and Penghu depressions in the western waters are studied in detail. The Cretaceous and Paleocene-Eocene source rocks are the major source rocks of these two depressions and peak gas generation begins at the burial depth near 2800 m. Study of the gas generation and petroleum systems indicates that these two depressions have good gas reservoiring conditions and are very important prospects for gas exploration in the western Taiwan Basin. The deep structural location of Nanri deression may have favorable conditions for accumulation of source-contacting gas.